1. Field of the Invention
The present invention relates to a steering control apparatus for a vehicle, and more particularly, to a steering control apparatus for controlling the turning angle of front wheels of a vehicle.
2. Description of the Related Art
Conventionally, a steering apparatus for front wheels of a vehicle, such as an automobile, is designed so that the turning angle of the front wheels is always in proportion to the turning angle of a steering wheel.
When the vehicle, mounted with this conventional steering apparatus, turns or corners at a speed higher than a predetermined level, the front wheels are first turned, so that a slip angle and cornering force (CF) are generated in the front wheels. Thereupon, a yaw moment is produced in the vehicle, so that the vehicle yaws, thereby also generating a slip angle in the rear wheels. As a result, a cornering force is also generated in the rear wheels, so that the vehicle can enjoy a reliable turning or cornering.
In the vehicle of this type, however, the body of the vehicle is subjected to a reactive force in the yawing direction when the front wheels are sharply returned from a cornering state to a straight-ahead state, in particular, so that the driving stability of the vehicle is lowered.
This phenomenon is attributable to the fact that a change of the cornering force of the rear wheels starts with a delay behind that of the front wheels, as shown in FIG. 11. Thus, after turning the steering wheel in a direction from the straight-ahead position (hereinafter, called as a first turning action), when the steering wheel is turned in the opposite direction form the turned position to the straight-ahead position (hereinafter, called as a second turning action), a reduction of the cornering force of the rear wheels delays behind that of the front wheel. In a transient stage of the second turning action, the cornering force of the rear wheels becomes much greater than that of the front wheels. These unbalanced cornering forces causes a reactive force in the yawing direction, as shown in FIG. 12, thus lowering the driving stability of the vehicle. This effect is noticeable when the steering wheel is sharply turned while the vehicle is running at high speed.